1. Field of the Invention
The present invention relates to an apparatus for the injection moulding of containers in plastic material, such as, e.g., single-dose bottles for pharmaceutical, medicinal and cosmetic products.
2. Description of the Related Art
Moulds are known for moulding containers composed essentially of a female matrix die wherein are obtained one or more cavities inside which can be inserted corresponding male elements, the cavities being in the same number as the containers moulded at each cycle.
Between the inner surface of the above cavities and the outer surface of the relevant male elements an inter-space is defined wherein is injected under pressure a plastic material to be moulded, of the thermoplastic, thermosetting type, etc.
The shape or the thickness of the inter-space defines both the shape and the thickness of the walls of the containers to be made.
In this respect, it must be noted that the thickness of the walls of the containers of the type of the single-dose bottles for pharmaceutical or cosmetic products is very thin, e.g., around 0.6÷0.8 mm, and requires very narrow tolerance intervals.
On the one hand in fact, the container must have characteristics of deformability and flexibility such as to allow the user to extract all the product contained inside, while on the other it must ensure the storage of the product in integral and unaltered conditions.
The moulds of known type are not without their drawbacks, including the fact that they do not guarantee the centring between the male element and the female matrix die.
The male element in fact, takes on misaligned and inflexed configurations with respect to the matrix die and this causes a change in the thickness of the inter-space defined between them and therefore in the thickness of the walls of the moulded container which, not only can take on values outside the acceptable tolerance interval, but can be interrupted by hollows and/or cavities.
Consequently, there is an increase in production rejects and, considering the retrieval of the rejected material is often difficult and costly, this results in obvious wastes of materials and energy, time extensions and increase in production costs.
To overcome these drawbacks, the document US 2001/0015512 describes the use of a particular injection-moulding apparatus having specific automatic centring means of the male elements in the corresponding matrix die.
Such centring means are composed of a pair of stems arranged inside corresponding holes obtained in the matrix die on opposite sides of each male element.
Inside the holes, the stems are moving closer to and away from the male elements between a backward configuration and a forward configuration.
In the backward configuration, in point of fact, the stems are positioned flush with the matrix die cavities and complete its profile, while in the forward configuration they are stopped up against the male elements, centring them and fixing them with respect to the matrix die and interrupting the continuity of the inter-space.
During moulding, the plastic material is injected into the mould until the inter-space is nearly totally filled (e.g., 90%) and, as long as this remains in fluid state, the stems are taken to the backward configuration restoring the continuity of the profile of the inner cavities of the mould.
In this configuration, the injection of the residual fraction of plastic material allows completing the moulding of the container walls.
This particular moulding apparatus is not without its drawbacks however including the fact that the alternate sliding of the stems in the corresponding holes of the matrix die soon determines the arising of serious problems tied to material wear.
The coupling between stems and holes, in fact, is achieved by adopting very narrow tolerances and the sliding is such as to cause fast surface wear, with the formation of small metal scraps which often end up inside the inter-space during moulding.
The containers thus formed therefore have inconvenient metal inclusions inside the walls which are visible from the outside, above all if the plastic material is white or transparent.
These containers inevitably end up by being rejected because not in compliance with the required project specifications from a technical and/or aesthetic and/or hygienic viewpoint.
In this respect, it must be underlined that in the medical, pharmaceutical and/or cosmetic field, the use of containers in imperfect aesthetic conditions produces in users the doubt that the fluid products contained in them have been contaminated and, consequently, their refusal to use them.
The apparatus according to the document US 2001/0015512, is also therefore susceptible to further upgrading aimed at reducing the rejects and production times, limiting material and energy waste, and cutting manufacturing costs, with the possibility of lowering the retail prices and making the products more interesting for consumers.